Double Trouble

In the last couple of years, with my wife and kids, I’ve visited many of the theme parks in Southern California and have been in their 3-D theaters because of my interest in stereoscopic imaging. I’m going to report on my experience, which is anecdotal and not a scientific survey. All the theaters were projecting stereoscopic movies using linear polarized light for image selection.

I’ll start with the Universal theme park which is just three miles from my house. I don’t mean to be casting stones, but I am bound to truthfully tell you what I saw. This isn’t science–it’s a limited sampling technique and I can’t come to any conclusion about the performance, on an on-going basis, with regard to any single theater. It’s the aggregate result that concerns me.

Just like you I’d would walk into the theater, put on the 3-D glasses, and look at the show. In all cases the theaters are using double projection. That is, they’re using a left and right projector with the method that, as far as I know, was first commercially used in the United States at the 1939 World’s Fair in Flushing Meadows Park in New York. It used two 35mm projectors that were interlocked. I don’t know if they were interlocked electrically or mechanically.

When I was a teenager I lived in Flushing and would ride my bike to Flushing Meadow Park and go skating at the ice rink. A nearby public library, a massive stone building, was originally a bathroom that was left to the city to use for books instead of toilets. Too much information? You be the judge.

I should back up a minute and say that I have set up many stereoscopic theaters and projection rooms using single and double projection equipment. I started using super-8 and 16mm projectors in the early ‘70s using timing belts that were geared to the driveshafts of the projectors. This allows for absolute control of synchronization and phase. It turns out that when you’re projecting stereoscopic movies the projector shutters also have to be phase. Of course, digital projectors have to have their fields in phase too. If you don’t do that, the image looks like it’s rippling and can drive you up the wall.

To get back to the Universal park at Universal City — we went to see SHREK 4-D. SHRED 4-D uses four projectors with edge-blending–a left and a right for the right half of the screen, and a left and a right for the left half of the screen. The projection was superb. I rate it as the best combination of content creation and projection I have seen in a park. Next time we returned to the park we looked at TERMINATOR 3-D, which I think got Jim Cameron started on his path of making stereoscopic movies. The movie is incredibly violent, harsh, and spectacular. The presentation is quite cunning, blending live action and multiple screens. However, on the front screen, which was the most important screen and which is on for most of the time, the projectors were out of synch or out of phase. It drove me up the wall. I don’t know how other people responded to it, but it was not a pleasant experience. If the left and right images aren’t presented simultaneously to within a specifiable tolerance, then action–especially rapid action, and “rapid action” could be an eye blink or just a gesture of a hand–becomes the damned funniest-looking thing you’ve ever seen. You don’t see anything like this in the visual world. The effect is kind of a gelatinous, weird, and peculiar look. It’s produced by a combination of spurious temporal parallax which has ramifications for depth due to the horizontal components of the combination of homologous points, and truly terrible results because of the generated vertical parallax components.

Next we went to San Diego, where my wife grew up and to SeaWorld where they were showing a pirate movie. The movie was well shot, and it would have been entertaining if projection hadn’t had several feet of vertical parallax. That means that the left and right images were offset up and down, requiring your eyes to fuse in the vertical direction. Your eyes aren’t meant to do that, generally speaking. When looking at a stereoscopic movie, you’d like be able to pass a horizontal line through the left and right image points.

The next theater I visited was Disneyland, and I saw HONEY, I SHRUNK THE AUDIENCE. The projection was perfect. No complaint. Everything looked great.

Finally, I visited LEGOLAND and their 3-D theater. The movie was about Lego racecar drivers. I think this was the only theater I went to that was using digital projectors–and the image would go totally out of phase from time to time. It’s hard to imagine how that could happen, unless it was built into the master, but every now and then a shot would appear pseudostereoscopic rather than stereoscopic. That means the left eye was seeing the right eye’s image and vice versa. This is not a pleasant viewing experience because he stereo effect is totally destroyed.

I’ve told you my experiences with looking at stereoscopic movies in theme parks where you’d expect there would be pretty good control of the projection. Alas, that isn’t always the case. In fact, based on my experience, it’s hit or miss. When projecting stereoscopic movies there are three major symmetries or congruences that need to which one needs to pay attention. If you can’t control these, it’s not worth the effort.

One general class of symmetries has to do with illumination and the brightness and color of the images, and also the evenness of the illumination fields. If one image is brighter than the other you may experience the feeling that one eye is “pulling.” Or you may get a headache. Next is a temporal symmetry. You want to have the images projected in synch and phase to within certain tolerances. The tolerance that I’ve determined experimentally, confirmed in the literature, is synch or phase must be maintained to within about 25% of the projection cycle for one frame. Finally, there is the geometrical symmetry that falls into two categories: alignment and magnification. You want to make sure that the left and right projection lens axes are crossed at the plane of the projection screen, and also you want the images to be of the same magnification. In additional, geometrical distortion, or keystoning, hard to control, needs to be eliminated or kept at an absolute minimum. And there must be no tilting of one frame with respect to the other.

It turns out that the symmetries are additive and combine to increase discomfort, and they are cumulative in the sense that the more you look at them the worse you feel. If you can’t control the projection, what happens is that the audience members will begin to feel eyestrain or uncomfortable. This kind of discomfort or pain can be referred to different parts of the body. For example, you may feel that your eyes hurt, or your head hurts, or you may feel nauseated or queasy. You don’t have any vocabulary for this experience because it doesn’t happen in the visual world; it only happens when you’re looking at a badly projected stereoscopic movie. None of this can be good for the medium of 3-D movies.

In order to make dual projectors work, and work as a product and not simply as a demonstration or stunt, there has to be a projectionist in the booth and not just at setup. Either that, or there has to be some major invention that automatically keeps the left and right fields symmetrical. I’ve come to believe that some of what is going on when projecting stereoscopic movies using two projectors might be explained by chaos theory: small initial differences in setup can lead to catastrophic failure after running for some time.

All of this is by way of explanation of why my colleagues and I at REAL D invented a single-projector setup that combines the left and right images so that they emerge from a single optical path. This is based on the projection system I designed for our scientific visualization customers we’ve been shipping for about 15 years, using an electro-optical switch that I dubbed the ZScreen®. By using a single projector, we are guaranteed that almost all of the symmetries are taken care of because the same optical path is used for both perspective views. The only thing that can’t be guaranteed–because this is a field-sequential process–is the temporal symmetry. But this is entirely mitigated by repeating the images at a high enough field rate. I have 20 years of experience showing images this way, and it works great.

Some of us care passionately about stereoscopic filmmaking and projection, a flame that has been kept alive for many years. I hate to see the flame extinguished with kluged double projection.